Thomas Aquinas

“ Therefore ingratitude is not always a mortal sin.
I answer that, As appears from what we have said above (A [2] ) , a man may be ungrateful in two ways: first, by mere omission, for instance by failing to recognize the favor received, or to express his appreciation of it or to pay something in return, and this is not always a mortal sin, because, as stated above (Q [106] , A [6] ) , the debt of gratitude requires a man to make a liberal return, which, however, he is not bound to do; wherefore if he fail to do so, he does not sin mortally. ”

“ In every sin there is material ingratitude to God, inasmuch as a man does something that may pertain to ingratitude. But formal ingratitude is when a favor is actually contemned, and this is a special sin. ”

“ Forgetfulness of a favor received amounts to ingratitude, not indeed the forgetfulness that arises from a natural defect, that is not subject to the will, but that which arises from negligence. For, as Seneca observes (De Benef. iii) , "when forgetfulness of favors lays hold of a man, he has apparently given little thought to their repayment."
Reply to Objection 3: The debt of gratitude flows from the debt of love, and from the latter no man should wish to be free. Hence that anyone should owe this debt unwillingly seems to arise from lack of love for his benefactor. ”

“ But sometimes it is impossible to repay a favor without sinning, for instance if one man has helped another to commit a sin. Therefore, since it is not a sin to refrain from sinning, it seems that ingratitude is not always a sin. ”

“ By committing a venial sin one is not ungrateful to God to the extent of incurring the guilt of perfect ingratitude: but there is something of ingratitude in a venial sin, in so far as it removes a virtuous act of obedience to God. ”

“ Further, no one should afford another an occasion of committing sin. But the ungrateful in receiving a favor is given an occasion of ingratitude. Therefore favors should not be bestowed on the ungrateful. ”

“ And yet ingratitude of this kind may happen to be a mortal sin, by reason either of inward contempt, or of the kind of thing withheld, this being needful to the benefactor, either simply, or in some case of necessity.
Secondly, a man may be ungrateful, because he not only omits to pay the debt of gratitude, but does the contrary. ”

“ Secondly, he should be inclined to turn his ungratefulness into gratitude, and if he does not achieve this by being kind to him once, he may by being so a second time. If, however, the more he repeats his favors, the more ungrateful and evil the other becomes, he should cease from bestowing his favors upon him. ”